1. Field of the Invention
This invention generally relates to laser scanning systems utilizing laser light sources for reading coded symbologies of different light reflectivity. More particularly, the invention pertains to laser scanning systems wherein adjustment of the intensity or spatial coverage of the beam is utilized to effect the detection and reading of coded symbologies.
2. Description of Related Art
Coded symbologies, such as bar code symbols, are commonly used in a number of applications to uniquely identify an item or group of items. Bar code symbols typically comprise a combination of black bars of varying widths alternating with white spaces of varying widths. A unique combination of bars and spaces represents the encoded information. Bar code symbols are typically affixed to an item using a label, or imprinted directly upon the item. The reading and decoding of bar code symbols is used to yield information concerning the item, such as a description of the item's destination, origination or particular characteristics such as color, price or quantity. Due to their inherent efficiency in tracking items, bar code symbols have been widely used for document tracking, inventory control, manufacturing control and employee identification.
Bar code symbol scanners are the devices used to read the bar code symbols to retrieve the desired information. The bar code symbol is illuminated by a beam of coherent light, such as a laser, that sweeps across the bar code symbol. The scanner detects the light reflected from the bar code symbol and determines contrasts between the bars, areas having a lower reflectivity, and the spaces, areas having a higher reflectivity.
An important consideration in using scanning systems of this type is the optimal orientation of the scanning beam with respect to the bar code symbol. It is preferable to have the scanning region cover the entire bar code symbol to ensure high accuracy. However, in many prior art scanning systems, the scanner provides only a single scan line and the bar code symbol must be precisely oriented in relation to the defined scanning region. Many current laser scanners of this type are adequate for bar code symbols which are presented in a "ladder" orientation wherein the bar code symbol resembles a vertical ladder as it moves by the scanner. In this configuration, the laser repeatedly scans from the top of the bar code symbol to the bottom as it passes the scanner. Even though the scan beam repeatedly scans vertically, the bar code symbol must still be oriented within the field of the view of the scan beam. The time consumed in precisely aligning the bar code symbol delays scanning and prevents high-speed bar code symbol scanning.
Bar code symbols may also be oriented in a "picket fence" orientation wherein the bar code symbol resembles a picket fence as it moves by the scanner. In applications where the bar code symbol is oriented as such, the scanner may be rotated 90 degrees to scan horizontally across the bars of the coded symbol. However, one problem with this approach is that the same horizontal portion of the bar code symbol is repeatedly scanned. Accordingly, a "no read" may result if the printing quality suffers at that portion of the code. Additionally, the bar code symbols must still be precisely aligned with the scanner.
Although there are some prior art systems that do not require as precise orientation, these systems have not proven to be wholly satisfactory. These systems have incurred various problems, including gaps in the scanning pattern, the requirement of a large minimum bar height, and a limited range of orientation. One such scanning technique includes redundant sweeps across the entire bar code symbol by the laser to assure an accurate reading of the bar code symbol. The redundant scanning avoids misreading of a bar code symbol due to possible localized width inaccuracies in the bar code symbol. However, no reconstruction of the bar code symbol is done. This method is best suited for reading bar code symbols which have been pre-positioned along the scan line of the sweeping laser beam.
It is not always possible to control the orientation or location of the bar code symbol with respect to the scan line of the scanning device. In applications where the orientation or location of the bar code symbol is not constant, two other scanner type options are available: the raster mirror wheel and the vibrating vane.
A scanner employing a raster mirror wheel projects multiple parallel scanning lines instead of a single scanning line. As a result, the raster mirror wheel scans more than one part of the bar code symbol and information corresponding to a part of the label which has poor print quality may be ignored. The raster mirror wheel is also beneficial when there is inconsistent placement of the codes since it provides a larger scanning region.
However, the scanning region for raster mirror wheels is still substantially limited. Raster lines are evenly spaced over a predefined area and the number of raster lines is limited to the number of facets on the mirror wheel. To prevent bar code symbols with small heights from passing between the raster lines, the distance between them must remain small. As a result, the scanning region, while larger than a single scan line, remains fairly small.
Use of a vibrating vane which is attached to a line scanner to create a variable raster can alleviate this limitation to some extent. The vibrating vane includes an additional mirror positioned in the beam path after the mirror wheel. The additional mirror continuously rotates a certain number of degrees, then reverses direction and rotates back to its origin. As a result, the number of scan lines is not limited to the number of facets on the mirror wheel and the vibrating vane has essentially an infinite number of raster lines. This makes the scanner more reliable in many picket fence applications and applications having inconsistent placement of the codes.
The vibrating vane, however, has substantial limitations. Bar code symbols with small bar heights require the scan lines to be close together. To compensate for this, either the sweep distance of the vane must be reduced or the frequency of the sweep must be reduced, thereby reducing the maximum conveyor speed. The orientation range of the bar code symbol is also limited to allow a scan line to pass through the entire code. As a result, tradeoffs must be made between minimizing bar code symbol height, maximizing conveyor speed and minimizing placement variation.
One current scanning system that requires less precise orientation is an "X" pattern scanner. An X pattern scanner with reconstruction software permits bar code symbols to be read in an omni-directional (i.e. 360.degree.) orientation, although the symbols are somewhat limited in that the scan pattern is fixed in size.
Thus it is desirable to have a scanning system which solves the numerous shortcomings associated with current scanning systems.